feat: defer reply of the LLM commands

[samhwang]

These commands have to be resolved within 3 seconds by default, but it's taking way longer because of how complex LLMs are, so we will need to defer the replies.

References:

• Deferred response: https://discordjs.guide/slash-commands/response-methods.html#deferred-responses
• Follow up response: https://discordjs.guide/slash-commands/response-methods.html#deferred-responses